An Efficient Ultra-Narrowband Yellow Emitter Based on a Double-Boron-Embedded Tetraazacyclophane

Ultra-narrowband and highly modifiable multiple resonance thermally activated delayed fluorescence (MR-TADF) materials are crucial for realizing high-performance wide-color-gamut display applications. Despite progress, most MR-TADF emitters remain confined to blue and green wavelengths, with difficulties extending into longer wavelengths without significant spectral broadening, which compromises color purity in full-color organic light-emitting diode (OLED) displays. In this work, we present a novel tetraazacyclophane-based architecture embedding dual boron atoms to remarkedly enhance intramolecular charge transfer through the strategic positioning of boron and nitrogen atoms. This arrangement induces a substantial redshift while maintaining structural rigidity and molecular orbital symmetry, with a hole-electron central distance of 0 Å, allowing for ultra-narrowband emission. The resulting MR-TADF material, HBN, delivers yellow emission peaking at 572 nm (2.168 eV) with an impressively narrow full-width at half-maximum (FWHM) of 17 nm (0.064 eV) in dilute toluene. Moreover, the corresponding phosphorescent-sensitized fluorescence OLED achieves yellow emission maximum at 581 nm, with a narrow FWHM of 25 nm, a high maximum external quantum efficiency of 36.1 %, and a luminance exceeding 40,000 cd m^-2. These outstanding photoluminescent and electroluminescent performances validate the superiority of our molecular design strategy, highlighting its significant potential for cutting-edge optoelectronic applications.